Study On Long-term Evolution Trend Of Global And Regional Aerosol Optical Properties And Its Driving Factors

Atmospheric aerosols,whether natural or anthropogenic,have important effects on earth's climate system on global and regional scales through aerosol–radiation interactions and aerosol–cloud interactions.Aerosol optical properties remain one of the largest uncertainties in assessment of aerosol radiative forcing and its climate response.With regard to the global and regional aerosol variations,carrying out the research of long-term trends in global and regional aerosol loading and the cause of formation through aerosol optical properties is currently the hot issue of aerosol climatic effects field.This study first assessed the performances of the MERRA-2reanalysis and MISR satellite-derived AOD products on global and regional scales using the AOD observations from both AERONET and CARSNET stations,and then based on the long-term AOD–related records obtained from MERRA-2 reanalysis and multi-satellite observations,comprehensively analyzed the evolution trends in total AOD as well as AODs for different chemical species,different particle sizes and non-spherical in different historical periods across the globe and 12 regions of interest?ROIs?,including northern China?NC?,southern China?SC?,northeastern Asia?NEA?,eastern United States?EUS?,western Europe?WEU?,southern Asia?SA?,Sahara desert?SD?,Middle East?ME?,northwestern China?NWC?,Amazon zone?AMZ?,central southern Africa?CAS?and southeastern Asia?SEA?.Then,based on revealing the impact of local emissions and meteorological factors on regional inter-decadal changes in regional AOD,the relative contributions of main driving factors that dominate the inter-decadal changes in regional AOD were quantitatively evaluated.Finally,with the CALIPSO-derived vertical profile of aerosol extinction coefficient?EC?and layered statistical detection sample data,a comprehensive analysis on the three-dimensional?3D?distribution structures,differences in regional vertical distribution and stratified change trends of different types of aerosols was carried out.The main conclusions were as follows.Results showed that the MERRA-2 AOD was comparable in accuracy to the satellite-retrieved AOD on the global scale?R=0.84,RMSE=0.14 and MAE=0.07?,but its performance has obvious regional differences.In regions such as eastern China and SA which are dominated by anthropogenic aerosols,MERRA-2 AOD is lower than the true value.This may be due to the fact that MERRA-2 is lack of nitrate and ammonium aerosols.Despite of this,it was revealed that,in general,MERRA-2 was able to quantitatively reproduce the AOD annual and seasonal trends?especially decadal trends?as observed by the MODIS/Terra.The evaluation results of MISR AOD showed that significant spatial agreement between MISR and ground-based AOD on the global scale,with an acceptable bias?r=0.85,MAE=0.06,RMSE=0.12and RMB=0.96?.Overall,about 80.4%of the MISR-retrieved daily AOD values fall within EE?1[±(0.05+0.20×AOD_{ob s})]of ground-based observations,and about 59.9%are within EE?2[±(0.03+0.10×AOD_{ob s})].Analysis of the time series of regional AOD during 1980-2016 found that non-monotonous decreasing trends were observed over EUS and WEU,but the intensity of this downward tendency has slowed over the recent decade.AODs in eastern China experienced a sustained and significant upward trend before?2006,and then the trend shifted from upward to downward due to the Chinese government's emissions-reduction policy.In contrast,SA showed overall significant positive trends throughout the study period,responding to the increasing anthropogenic emissions in the region.The statistical analyses suggested that the meteorological parameters explained a larger proportion of the AOD variability?20.4%–72.8%?over almost all ROIs during 1980–2014 when compared with emission factors?0%–56%?.In anthropogenic-aerosol-dominant?AAD?regions,SO₂ was the dominant emission factor,explaining 12.7%–32.6%of the variation in AOD.In biomass-burning-dominant?BBD?regions,black carbon or organic carbon was the leading factor,contributing 24.0%–27.7%of the variation.Additionally,wind speed was found to be the leading meteorological parameter,explaining 11.8%–30.3%of the variance over the mineral-dust-dominant regions,while ambient humidity?including soil moisture and relative humidity?was the top meteorological parameter over the BBD regions,accounting for 11.7%–35.5%of the variation.The results of this study indicate that the variation in meteorological parameters is a key factor in determining the inter-decadal change in regional AOD.Inter-annual variations in regional AODs for different aerosol species indicated that sulfate aerosol?SO₄?was the most important chemical component driving the decline in total aerosols over EUS and WEU,but the increases in carbonaceous and natural aerosols?including dust and sea salt?may offset the decline of total aerosol to a certain extent.In SD and ME,mineral dust was the dominant aerosol type driving the inter-annual evolution of total AOD,but the contribution of anthropogenic aerosol in ME was growing yearly.Anthropogenic aerosols?especially SO₄?were the leading aerosol type that dominates the inter-annual evolution of total AOD over NC and SC.It is worth noting that the decrease of total AOD in the above regions after 2006 was not only due to the significant decline in SO₄,but also the decrease in other types of aerosols has contributed to varying degrees.In SA,the inter-annual variation of total AOD was driven by both anthropogenic and natural aerosols,and the leading role of the former was increasing year by year.In addition,the evolution trends of different types of aerosols retrieved by MISR further indicated that in most land regions dominated by anthropogenic aerosols,the spatial distribution of trend in small-size aerosols?diameter<0.7?m?was almost the same as that of the total aerosol.The above results indicated that the small-size aerosols produced by anthropogenic activities were the main aerosol type that drives the change of total aerosols in the regions dominated by anthropogenic aerosols.Analysis of the CALIPSO-derived 3D vertical structure of different types of aerosols shows that the enhancement of vertical EC for dust aerosol mainly occurred in near-surface layer?below 3 km?of the dust source area,and that EC could exceed0.1km^-1 in the range of 0-1 km altitude.Polluted dust aerosol was widely distributed,and was mainly in dust areas and its downstream regions with intensive anthropogenic activities.The maximum value of elevated height for polluted dust could exceed 4 km.Smoke aerosol affected the entire globe,and its elevated height could exceed 6 km in the ocean area.In the globe and all 12 ROIs,more than 50%of the columnar aerosol loading was located the lower troposphere?0-2km?,and the contribution ratio over 6km was less than 2%.The land aerosol could be lifted to a higher altitude under the influence of topography and atmospheric circulation.Evaluation of the all layer and stratified frequency-of-occurrence?Fo O?of different types of aerosols indicated that on a multi-year time scale,the global average Fo O for all aerosol types was 5.6%,of which clean marine,dust,polluted continental,clean continental,polluted dust,smoke,and dusty marine contributed 2.08%,1.08%,0.41%,0.10%,0.86%,0.64%,and0.48%,respectively.The regional mean maximum values of the entire layer of Fo O for all types of aerosols occurred in the ME?21.0%?,while the minimum values occurred in WEU?6.3%?.The regional average maximum value of Fo O for polluted dust was located in NC,which is about 5.13%.The global and regional stratified trends in AODs for different types of aerosols?i.e.dust,polluted dust and smoke?suggested that significant?P<0.1?decreasing trends for dust aerosols were observed over NC,SEA and ME,with the decadal decreasing rate of-38.02%,-20.34%and-23.05%,respectively.Polluted dust showed a significant decline over NEA,NC,and SC,and a significant increase over SA and ME.In contrast,smoke aerosols have shown a significant downward trend in most regions of the world,with the decadal decreasing rate of-13.29%,-13.51%,and-12.89%for global,land and ocean,respectively.In addition,the layered AOD of different types of aerosols played different roles in driving the evolution trend of total AOD.For example,in the AAD regions?such as eastern China,SA and EUS?,the trends pattern of total AOD was basically consistent with that of integrated AODs between 0 and 3 km,indicating that the changes in anthropogenic aerosols accumulated in the lower layers dominated the changes in total aerosol in these areas.